Art of finishing ferrous metal



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This invention relates tothe m of finishing Fntenten "rimericanchemlcalPaint Company, Ambler,

Yo a corporation of Delaware No Drawing. Continuation of applicationSerial No. 743,839, September 13, 1934. This application July 9. 1935,Serial No. A69

acclaim. (c1, mic-0.5)

resulting color will depend on the temperature re s PATENT- oFF cEierrous metal and its principal object is to provide a final finish thelife of which is materially lengthened due to greatly increasedresistance to the destructive effects of corrosion or mechanical etrmn,

A further object of the invention is to provide a ferrous metal surfacewith a coating of paint, varnish, lacquer, japan or the like, the lifeof which coating is greatly increased and its adhesion promoted,especially, as stated, where it may be subjected to atmosphericcorrosion or mechemical strain.

An eddiuonal object is to produce finishes of very high luster with verythin coatings of paint or the like.

Still another object is the provision of e. proc ess for the g oimetallic surfaces which will avoid the formation of hard, adherentincrustetions on the vet, hosting coils or other equipment used intreating the metal especially where this equipment is made of steel orother corrodible metal. Y

Another object is to increase the longevity of the protection eifordeolby temporary finishes such as oil, grease, was, water-point, white-wash,etc. i r In connection with all oi the foregoing objects it is a furtherimportant object oi. my invention 39 to remove oil end grease from thesurface of the metal while simultaneously providing it with 9.

- peint presewetive coating. 5

Attention is called to the fact that this application is e, continuationof my earlier application Serial No. l lSfiSQ, filed September 13, 1934.

K have discovered that chromic acid in solution may he activated so asto produce the desirable efiect of increasing the longevity of finishesap plied to ferrous metal surfaces.

Broadly speaking, the invention consists in treating the metallicsin-feces to co finished with a solution containing chromite sold andone or more of o number of activating substances to be and time oftreatment as well as upon the composition and concentration 01 thesolution.

Generally stated the activating substance is an anion which actscatalyticelly to increase the 5 action of the chromic acid solution onthe. metal.

In my .search for activator anions I have found that chloride, bromide,iodide, fluoride and ferricyanide anions are highly efiective..Ferroline earth metals, iron, zinc, cadmium, copper,

mercury, etc, is not detrimental, although none of these ions isessential to the operation of the process, nor do any of them act asactivator ions. The essential ingredients of my improved treetingprocess are chromic acid and anions of the activator type abovedescribed by which are meant helogenions, ierricyanides, end otherswhich produce my desired result.

' I have found, in general that the desired effect can be obtained fromsolutions which contain at least .(l3 gram equivalents of activatoronions per liter. Furthermore, the-most generally applicable solutionalso contains free chromic acid, although in certain instances thepresence of dichromotes of certain metelssuch as zinc, iron, etc,furnishes sumcient acidity to operate the process and in such cases onlythe minimum quantity of activator onions need be present. However,solutions in which no free chromic acid is present are primarily usefulonly in instances where the surface to be treated is thoroughlydegreased.

In the more generally applicable solutions, which contain free chromicacid, the concentration of activator onions should he at least twotenthsthe concentration of the free chromic acid by weight. Four times as muchactivator anion as of free chromic acid or more may be used, butexcessive amounts oi activator anions lead to a decrease in theeffectiveness of the treatment and I prefer to use activator anions at aconcentration of from two-tenths to four times those of .the freechromic acid, provided that a minimum of .03 gram equivalents ofactivator anion per liter is present.

Chromic acid behaves as a monobasic acid to ward BromfiresolGreenindicator, the dichromates oi the alkalies being barely alkalineand zinc dichromateibarely acid toward this indicator. For the purposeofthe present disclosure, therefore, I consider the "free chromic acidvin the solution to be that quantity of the chromic acid present which iscapable of being titrated with caustic soda solution before thedevelopment of a green color when using Brom Cresol Green as theindicator. Combined" chromic acid is considered to constitute all of thechromic acid presout "not free as defined above.

As a specific example, I will describe my improved process as it may beapplied to the coating of a steel stamping, such as an automobiletender.

The stamping as it comes from the press is generally contaminated with:mill oil, grease, and/or drawing compound, and these impurities may beremoved by any of the well known conventional cleaning processes if sodesired, but the-process of my invention makes it possible to eliminatesuch a precleaning step. If precleaning is resorted to, scale and rustmay be removed with an acid pickle followed by washing with water, andoil and grease maybe removed by However, if the stamping does not bearobjectionably heavy rust or scale, it is unnecessary to resort to such aprecleaning step as stated above, provided that the treatmenthereinafter described is thoroughly eilected. My treatment will removeoil and grease and even though scale or rust are not removed, I havediscovered that it will produce very satisfactory paint life without thenecessity of any precleaning step. In the case oi scale or rust thetreatment apparently removes or renders harmless the corrosionstimulative materials and while tightly adherent scale or rust mayinterfere somewhat with the smoothness of the final finish, it will notstimulate further corrosion.

Furthermore, it should he noted that my improved process is quiteinsensitive to any precleaning method which may be. used because the@103, since chromic acid, HzCIOc, is not a stable,

weighable compound:

. Formula N0. 1

Chromic acid (010:) lb 0.033, Common salt lb 0.167 Water, to make"gal..- 1.000

sumcient of the solution'of the foregoing formula is prepared to fillthe tank of one section of a conventional metal-washing machine, andpreferably heated to a temperature between about and 200 F. andpreferably in the neighborhood of 180 F., although the solution may beemployed at any temperature up to its boiling point, to which end thetank is provided with some suitable means of heating, such, for example,as steam coils.

I prefer to wash the treated surface before applying the final finishbecause I find that to do so improves the character of the To this end,therefore, a second section of the wash= ing machine is filled withwater which may be cold or may be heated to accelerate the drying of thewashed stampings. The stampinzs are then passed through the washingmachine by a conveyor orother suitable means, the pumps of the washingmachine being operated to keep jets of the solution and the waterimpinging strongly on the stamplngs in the respective sections.

The stampings should be subjected to the impinging action of thesolution in the first stage of the washing machine for approximately- 1'minute or thereabouts although the time re- I, is without eifect whenthe treatment has been properly performed in accordance with thisdisclosure.

The treatment is of distinct advantage over prior practices familiartothe art especially where equipment or a corrodible nature is employed,

such as a steel vat with heating coils, because with my improved processno serious incrustation of the vat, heating coils or other equipmentoccurs. This is a distinct advantage over processes employing solutionsof phosphates in which aheavy stone-like incrustation soon covers thevat and the heating coils or other equipment, which incrustation, ofcourse, interferes greatly with the transmission of heat through thesolution and necessitates frequent draining and has a tendency to causea certain degree of corrosion although this is not so serious as topreclude its use in metallic apparatus oi this mud. However, suchcorrosion entails a useless consumption oi chemicals and where metallicvats cleaning of the vat. It should be noted. however, thatin the-caseof steel equipment, the solution and/coils are employed, it ispreferable to construct them of some non-corrodible material such asstainless steel.

After the treatment in the washing machine the stampings are dried. Thismay be accomplished by any convenient means, but it is prelerable to drythe stampings at an elevated temperature, as for instance, in an ovenwhich may which will cause oxidation or the steel with the be maintainedat any temperature below that consequentiormation of a scale. When drythe just described is quite vigorous in its action and in order toreduce the violence of this action as well as to bullet the solution andto furnish an auxiliary supply of potentially "free" chromic acid Iprefer to add to the solution, in addition to the free chromic acid andactivator anion,

aiiaici chromic acid combined with a metal or metals capable of formingwater insoluble normal or basic chromates.

Such solutions will then contain chromate ions, activator anions,sufiicient hydrogen ion to be capable of titration as free chromic acid,in accordance with the above definition, and ions of a metal or metalswhich are capable of forming insoluble basic or normal chromates.

Among the metals whose dichromates have been used to successfully bufferthese solutions are zinc, cadmium, calcium, aluminum, iron, chromium,manganese and, copper. Solutions prepared in this way have the furtheradvantage that they produce the desired effect on the longevity andadhesion of applied finishes with less attack on the metal undergoingtreatment and this results in economy of chemicals consumed and reducesthe frequency oitesting and of additions of reagents.

A solution, buffered as described, is given below:

Formula No. 2

Chromic acid (CrOs) lb.. 0.042 Zinc dichromate lb 0.202 Sodium chloridelb' 0.083 Water, to make gal 1.000

This solution is ready for use, although for economy in shipping andstorage it may be prepared in concentrated form by mixing the chromicacid with about .15 gallon of water and stirring in zinc oxide andsodium chloride gradually, in the proper proportions, until dissolved,after which the concentrated solution may be diluted to conform to theabove formula before it is used.

Another useful formula is given below:

- Formula N0. 3

Chromic acid; "1b-; 0.0500

Ferric chloride "lb-.. 0.0700 Common salt lb 0.0750 Water, to make gal1.0000

As mentioned above, certain types of metal surface may be processed insolutions containing no free chromic acid. In these solutions thechromic'acid can be considered to be present as the dichromates of oneor more metals.

of at least 0.03 gram equivalents per liter, although concentrations ashigh as 0.10 gram. equivalents or more are permissible. However, theresults obtainedare less satisfactory when the concentration ofactivator anion exceeds 0.07

gram equivalents per liter. These solutions produce practicallyeffective results only when the metals are entirely free of grease orother contaminants when treated. A solution containing no free chromicacid which is suitable for treating grease-free steel, may be made asfollows:

Formula; No. 4

Chromic acid ..lb 0.2500 Zinc oxide lb 0.1067 Sodium chloride 1b 0.0333Water, to make gal- 1.0000

Activator anions must be present in a concentration of water. Forinstance, the solution of Formula No. 1 may be prepared as follows:

Eormula No. 5 I Chromic acid (CrO3) 1b 0.333 Common-salt lb 0.667

The above dry admixture may be conveniently and economically shipped insealed containers. When it is to be used it is dissolved in enough waterto yield four gallons of a solution identical with that of Formula No.1.

Another method of preparing a dry admixture is to evaporate to dryness aconcentrated solu-- tion such as that given under Formula No. 2. Theresulting residue, reduced to particles of convenient size, serves, onsolution in the appropriate amount of water, to reproduce the solutionof Formula No. 2.

The solutions of any of the above formulas may be applied by any methodwhich will cause them properly to wet the surfaces to be treated.However, where no precleaning step is employed more or less vigorousmechanical treatment may be found necessary to achieve thorough wettingof the surface of the metal, and this, of course,

. will depend somewhat upon the nature and ouchtity of the contaminatingmaterials which are present. For instance, the treating solution may beapplied by flowing, brushing, mopping, or wiping oi the surface with orsubmergingit in the solution as well as by impingement of jets of thesolution in a washing machine as described above.

In case such procedures are adopted, certain ob vious precautions maybecome necessary. its the speed of any chemical process is largelyaffected by the temperature, it will be necessary to increase the timeof treatment as the eflective temperature of the surface is lowered.This is exemplified by the directions under Formula No. 2. It issometimes useful when the treatment of the surfaces is carried out inthe cold to use solutions more concentrated than those given in theexamples. Such changes, however, will be obvious to one skilled in theart.

In addition to its value in producing longlived finishes generally, myprocess has the advantage of malringpossihle the production of finishesof very high luster with very thin coatings of paint, varnish, lacquer,japan or the like. Moreover. incidental scratches or other injuries tothe finished surfaces are prevented from spreading under the organiccoating material. l' iuthermore, as stated above, the treating processof this invention is useful in increasing the longevity of theprotection afforded by temporary finishes such as oil, grease, wax,water, paint,

whitewash, etc. which are often applied during fabrication or whileawaiting the application of like.

' the class which consists of zinc, cadmium, cal-" 2. The method offinishing the surface of fer- 'rous metal which compflses forming achromate a finish of paint, varnish, lacquer, japan, or the 3. Themethod of finishing the surface of ferrous metal which comprises forminga chromate coating thereto. by treating it with a solution containingfree chromic acid and activator anionsfrom the class which consists ofhalogens and ferricyanides, the concentration of the activator anions inthe solution being from about twotenths to four times the concentrationof the free chromic acid, by weight, but not less than .03 gramequivalents per liter, and applying to the coated swim a finish ofpaint, varnish, lacquer, japan, or the like.

4i. 'lhe method of claim 2 wherein the solution also contains dichromteof at least one metal of the class which consists of zinc, cadmium,calcium, aluminum, iron, chromium, manganese, and copper. 5. The methodof claim 3 wherein-the solution also contains dichromate of at least onemetal of cium, aluminum, iron, chromium, manganese, and copper;

7 ii. The method of finishing the surface of fer rous metal whichcomprises forming a chro'mate coatingthereon' by treating it with asolution containing free chromic acid and sodium chloride, theconcentration of the chloride ion from the sodium chloride, being fromabout two-tenths to four times' the. concentration of the free chromicacid, by weight, but not less than 0.03 gram-equivalents per liter,washingand drying the coated surface, and then applying a finish ofpaint, varnish, lacquer, japan, or the like.

7. The method of finishing the surface of ferrous metal which comprisesforming a chromate coating thereon by treating it with a solutioncontaining free chromic acid, zinc dichromate and sodium chloride, theconcentration of the chloride ion from the sodium chloride, being fromabout ttvo-tenths to four times the concentration of the tree chromicacid, by weight, but not less than @103 grem=eouivalents per liter,washing and drying the coated surface. and then applying a finish ofpoint, we s-a t lacquer, japan, or the like.

8. The method lei-finishing the surface of iron or steel comprisesforming a chromate coating thereon hy treating it with a solution ofapproximately the following composition:

Chromic acid (@rOal lb. 0.083 Common salt lb 0.167 Water, to make gal1.900

washing and drying the coated surface and then in applying a finish ofpaint, varnish, lacquer, japan, or the like.

9. The method of finishing the surface of iron consist or steel whichcomprises forming a chromate coating thereon by treating-it with asolution of. approximately the following composition:

Chromic acid (CrOa) 1e o.es2 Zinc dichromate ..s lb c202 Sodium chloridelh 0.083 Water, to make "gal" 1.0M

washing and drying the coated surface, and then applying a finish ofpmnt, varnish, lacquer, japan, or the like.

10. The method of finishing the surface of iron or steel which comprisesforming a chromate coating thereon by treating it with a solution ofapproximately the following composition:

Chromic acid (@r03) lb 0.2500 Zinc oxide lb 0.1067 Sodium chloride lb6.0333 Water, to make "gal" 1.0000

- thereof a finish of paint or the like applied to an integral chromatecoating having the composition resultingfrom treating the article with asolution containing free chromic acid and activator anions from theclass which consists of halogens and ferricyanides wherein theconcentration of activator anions in the solution is between abouttwo-tenths and four times the concentration of the free chromic acid, byweight, but not less than .03 gram-eqmyalents per liter of the treatingsolution.

14. A ferrous metal article having on the surface thereof a finish ofpaint or the like applied to an integral coating of the compositionresulting from the treatment of claim 6.

15. A ferrous metal article having on the sur face thereof a finish ofpaint or the like applied to an integral coating of the compositionresulting from the treatment of claim 7.

16. A ferrous metal article having on a surface thereof a finish ofpaint or the like applied to an integral coating of the compositionresulting from the treatment of claim 8.

17. A ferrous metal article having on a surface thereof a finish ofpaint or the like applied to an integral coating of the compositionresulting from the treatment of claim 9.

18. A ferrous metal article having on a surface thereof a finish ofpaint or the like applied to an integral coating of the compositionresulting from the treatment of claim 10.

GER-AID C. ROMIG.

